1. Field
Embodiments of the present invention may relate to a method of preparing biodegradable polyester polymer materials both in a filament type and a sheet type using a compressed gas. Embodiments of the present invention may relate to a method of preparing biodegradable polyester polymer materials both in a filament type and a sheet type, which comprises the following steps: 1) preparing biodegradable polyester polymers through a solution-state polymerization process of a cyclic monomer using a compressed gas as a reaction solvent in the presence of a catalyst; 2) performing a flash-spinning process of the biodegradable polyester polymers prepared above in order to form a polymer material in a filament type; and 3) performing a calendering process of the polymer material in a filament type prepared above in order to form a polymer material in a point-bonded sheet type.
2. Background
The basic flash-spinning process of making flash-spun nonwoven products, specifically Tyvek® spunbonded olefin, was first developed more than twenty-five years ago and put into commercial use by DuPont (E.I. du Pont de Nemours). During that time, DuPont developed two basic forms of a surface-bonded material and a point-bonded material for flash-spun nonwoven products. The surface-bonded material is prepared by thermally bonding flash-spun nonwoven products with traversing the surface of a sheet. Further, the point- or pattern-bonded material is prepared by thermally bonding flash-spun nonwoven products to a point or pattern forming the part of a sheet showing a much more or weaker binding affinity. While the surface-bonded material is typically stiffer than the point-bonded material and has a paper-like hard structure, the point-bonded material is more flexible and has a fabric-like soft structure.
The general flash-spinning apparatus is disclosed in U.S. Pat. No. 3,860,369 issued to Brethauer et al. Further, the process of forming plexifilamentary film-fibril strands and forming the same into a nonwoven sheet material has been disclosed and extensively discussed in U.S. Pat. No. 3,081,519 issued to Blades et al., U.S. Pat. No. 3,227,794 issued to Anderson et al. and U.S. Pat. No. 3,169,899 issued to Steuber (all of which are assigned to DuPont). This process and various improvements thereof have been practiced by DuPont for a number of years in manufacturing its Tyvek® spunbonded olefin.
A degradable polymer material has been in the spotlight in various fields of medicine, agriculture, environment and the like due to its specific degradable characteristics. The biodegradable polymer is roughly divided into a natural biodegradable polymer and a synthetic biodegradable polymer. Among them, the natural biodegradable polymer, which is made of natural materials, has several advantages such as high affinity to the environment, high physical properties, adaptability to a living body and the like. However, the natural biodegradable polymer suffers from disadvantages since it is expensive and cannot be arbitrarily controlled due to its natural characteristics. Meanwhile, the commercial value of synthetic biodegradable polymer has been constantly growing since its characteristics can be artificially regulated to make up for the weak points. This is clearly different from the natural biodegradable polymer.
Among the synthetic biodegradable polymer materials, polylactide (cPLA) exhibits good physical properties as well as high compatibility and innoxiousness to the environment. Thus, it has received a considerable attention in the environmental and medical fields. Especially, the synthetic biodegradable polymer material has been efficiently used in the environmental field such as a disposable wrap film, an agricultural and industrial film and a food packaging container, as well as in the medical field such as a drug delivery system (DDS), a pin, a screw and a suture for fixing the bone and tissue, etc.
The synthetic biodegradable polymer with high molecular weight is generally prepared by a solid-state polymerization process using only a monomer and a catalyst in the absence of a solvent, wherein the monomer is polymerized at a temperature below the melting temperature of the polymer. However, the solid-state polymerization process has several problems such as non-uniformity of physical properties, adulteration of low molecular weight materials and workability deterioration. In particular, in order to apply a polymer prepared in the large-scaled solid-state to a molding fabrication process, the polymer should be crushed into a small size. This makes the preparation process complicated during the mass-production of a commercial scale and increases the production costs.
In order to overcome these problems, there have been reported several methods of preparing a polylactide copolymer, which is useful as a biodegradable and biocompatible material by a ring opening polymerization method using alkyl lactate monomer, a stabilizer, a metal compound catalyst and supercritical carbon dioxide as a dispersion medium (Hile, D. D. and Pishko, M. V., Macromol. Rapid Commun. 20: 511-514, 1999; Hile, D. D. and Pishko, M. V., J. Polym. Sci. Part A: Polym. Chem. 39: 562-570, 2001). Since the polylactide copolymer prepared according to the above method is insoluble to supercritical carbon dioxide, a fluoropolymer must be inevitably used as a stabilizer. However, although the polymerization is conducted for 48 to 72 hours, since the molecular weight of the polymer prepared by said method is at most in the range of 28,000 to 30,000 g/mol, there is a problem in that the molecular weight of the polymer is extremely low for the long reaction time. Further, it is cumbersome since the additional process of removing the stabilizer must be carried out after the reaction is completed.
Also, carbon dioxide as the supercritical fluid is frequently used due to its low critical temperature and critical pressure, low costs, incombustibility and innoxiousness. However, the supercritical carbon dioxide cannot dissolve polymers, except for fluoride-containing polymers and silicon-containing polymers (e.g., siloxane). Hydrocarbons and hydrochlorofluorocarbons (HCFCs) are known to optimally dissolve various polymers with high molecular weight when being used as a solvent.
In order to resolve the aforementioned problems of the conventional methods for preparing a biodegradable polyester in the solid-state process or using the supercritical carbon dioxide, the present invention has developed a method of preparing biodegradable polyester polymers both in a filament type and a sheet type. This is achieved by: preparing polymers with high molecular weight in the particle form within a short reaction time using a compressed gas such as hydrofluorocarbons (HFCs), HCFCs, dimethylethers and the like, which are in a supercritical state and can be used as a reaction solvent for a solution-state polymerization process; and performing flash-spinning and callendering processes of the polymers prepared above in a reactor as a single consecutive process to form polymer materials both in a filament type and a sheet type.